An extensive research and development has been conducted on the use of lithium metal, which is capable of realizing high voltage and high energy density, as the negative electrode of non-aqueous electrolyte secondary batteries. This has lead to the current commercialization of lithium ion batteries that use a graphite material in the negative electrode, which material reversibly absorbs and desorbs lithium and provides good cycle life and safety.
However, the useful capacity of batteries using a graphite material-based negative electrode is approximately 350 mAh/g, which is very close to 372 mAh/g theoretical capacity of the graphite material. Therefore, as long as a graphite material is used in the negative electrode, it is not feasible to achieve a dramatic increase in capacity. Meanwhile, as the functions of portable appliances are becoming more and more sophisticated, non-aqueous electrolyte secondary batteries used as the energy source of such appliances are required to have higher capacities. Accordingly, in order to achieve higher capacities, negative electrode materials having a higher capacity than graphite become necessary.
Alloy materials containing silicon or tin are currently receiving attention as the materials that offer a higher capacity. Metal elements such as silicon are capable of electrochemically absorbing and desorbing lithium ions, thereby enabling a very-large-capacity charge/discharge in comparison with graphite materials. For example, it is known that silicon has a theoretical discharge capacity of 4199 mAh/g, which is 11 times higher than that of graphite. For example, Japanese Laid-Open Patent Publication No. 2002-83594 discloses a non-aqueous electrolyte secondary battery including a negative electrode that has a silicon thin film on a current collector. Also, Japanese Patent No. 2997741 discloses a non-aqueous electrolyte secondary battery that uses as an active material a silicon oxide, which has a lower capacity but offers a longer life than silicon.
However, an alloy material capable of electrochemically absorbing and desorbing lithium, such as silicon or a silicon oxide, has a very large irreversible capacity when used as a negative electrode active material. If the irreversible capacity is compensated for with lithium from the positive electrode, the positive electrode active material that cannot contribute to charge/discharge reactions increases, so that the capacity of the battery itself decreases.
Further, as described above, graphite is used as a negative electrode active material in common lithium ion batteries, and graphite also irreversibly loses capacity when it reacts with a non-aqueous electrolyte to form a film. Usually, this irreversible capacity is also compensated for with lithium from the positive electrode and, hence, the battery capacity decreases relative to the discharge capacity inherently delivered by the positive electrode.
In order to compensate for the irreversible capacity, for example, International Publication No. WO 96/27910 discloses affixing a lithium-based metal foil to an electrode assembly composed of negative and positive electrode sheets that are spirally wound together with a separator. This documents discloses that by aging the electrode assembly affixed with the metal foil after the injection of an electrolyte, lithium is preliminarily inserted into the negative electrode.
Further, Japanese Laid-Open Patent Publication No. 2005-38720 proposes forming a negative electrode mixture layer on a negative electrode current collector and forming thereon a light metal layer made of lithium metal by a dry film formation method such as vacuum evaporation or ion plating. This document discloses that by storing the negative electrode with the light metal layer in a dry atmosphere or an electrolyte, lithium ions are absorbed in the negative electrode mixture layer.
However, according to the approach of the above-mentioned WO 96/27910, lithium ions are unevenly diffused in the negative electrode after the aging, since the negative electrode has an area that is in contact with the lithium-based metal foil and an area that is not. Hence, when charge reaction is caused, lithium metal is deposited in the area of the negative electrode active material layer where excessive lithium ions are present. The deposited lithium metal eventually forms dendrites, thereby causing a trouble such as an internal short-circuit of the battery.
Also, according to the approach of the above-mentioned Japanese Laid-Open Patent Publication No. 2005-38720, the whole surface of the negative electrode can be evenly covered with lithium, unlike the approach of WO 96/27910. However, in the case of using vacuum evaporation, in particular, when lithium steam solidifies on the surface of the negative electrode active material layer, the negative electrode active material is subjected to the heat of solidification. Since this heat of solidification is very large, most binders made of, for example, an organic polymer are decomposed and become deteriorated due to heat.
Furthermore, it is widely known that an electrochemical process is applied to the negative electrode active material layer affixed with the lithium metal film, in order to compensate for the irreversible capacity. In this case, however, reaction tends to proceed unevenly, so that expansion due to the absorption of lithium becomes uneven, which may result in poor current collection.